Applicant's co-owned U.S. Pat. No. 4,927,125 discloses a set-up assembly with upper and lower plates hingedly connectable and disconnectable along two spaced parallel axes. Applicant expressly incorporates U.S. Pat. No. 4,927,125 by reference herein, in its entirety. Aligned with each axis, the set-up assembly includes a pair of axially extendable and retractable center pins mounted on one of the plates which may be quickly and easily moved along the axis into and out of engagement with the other plate. The use of extendable and retractable center pins to accurately hold the upper plate to the lower plate simplifies set up operations and reduces the operator's set-up time for those machining operations which require hinged raising of either end of the upper plate.
The extendable and retractable center pins also facilitate machining operations which require simple, fast and effective holding of a pallet to a support plate. With a plurality of pallets, a workpiece mounted to each pallet, and a machining device located adjacent a lower plate in an operating position, the workpiece-holding pallets can be connected and disconnected to the lower plate in assembly line fashion to reduce machining time and expense.
With this set-up assembly, the center pins are spring-biased inwardly toward the pallet. To retract the pins, the pins are pulled outwardly against the spring force and then rotated perpendicularly into a latched position. For larger size plates, i.e., ten inches by twelve inches, the spring force is generally much higher, so that the upper plate can be held securely in position for machining operations. Unfortunately, for upper plates of this size, the high spring force makes it difficult to manually retract the pins outwardly from engagement.
Moreover, because of the structural relationship of the pin, the spring and the lower plate, this structure does not lend itself particularly well to automated placement, securement and removal of the upper plate to a workstation. This is due primarily to the susceptibility of pin or pin component breakage if the pin is retracted too far or rotated at the wrong time.
Also, this prior structure is not particularly suited to use with some types of work-holding operations. Often, the size and/or shape of the part, or workpiece, determines the manner and the orientation in which it should be held during a machining operation. For example, many larger parts require the use of one or more clamps, in combination with other devices mounted to a work table or pallet. The machining operation may also dictate that the part be held above a work table a predetermined distance, or, in a sense, held in three-dimensional space.
The greater the number of mechanisms required to hold the part, the more difficult and time consuming set up operations become. As a result of this increased difficulty and additional time, it also becomes more difficult to achieve accuracy and repeatability in holding such parts, particularly odd shaped parts and/or parts which must be held above a surface to allow a machining operation to be performed. Also, for some parts, the use of clamps, which apply compression to the held part, cause distortion during the machining step. This adversely offers the accuracy of the final machined part.
It is an objective of this invention to facilitate the automating of set up operations for accurately and repeatably holding a workpiece-holding pallet or workpiece at a work station for the performance of a machining operation on the workpiece.
It is another objective of the invention to facilitate the extension and retraction of center pins used to engage a relatively large size workpiece-holding pallet or workpiece at a workstation.
It is still another objective of the invention to increase versatility in the accurate and repeatable holding of a workpiece or a workpiece-holding pallet at a workstation, thereby to accommodate various sizes and shapes of workpieces and/or workpieces which must be held above a work surface during performance of a machining operation.
This invention meets the above-stated objectives by utilizing an extendable and retractable connector which rotates into and out of engagement with a workpiece-holding pallet, or the workpiece itself. More particularly, the connector includes a pin, an actuating knob or handle for rotating the pin and a wrap spring which interconnects the pin and the knob in a slippable manner. The wrap spring acts as a torque limiting device to control the pressure in one direction, and the wrap spring acts as a clutch in the opposite direction.
The pin mounts within a bore machined in a lower plate at a workstation. An external surface of the pin which is located within the bore includes a spiral cam which coacts with a radially inwardly directed cam follower that is rigidly connected to the lower plate. The cam follower resides within and traverses the cam so that the pin will extend or retract upon rotation of the actuating knob. During rotation of the knob to extend the pin, the wrap spring will slip if the knob is rotated too far. Thus, the connector acts as a torque limiting device to limit the rotational force applied to the pin once the pin is fully extended. This construction assures that an automatic rotating mechanism such as a stepping motor will not inadvertently break the pin during extension by accidentally rotating too far. Moreover, this construction assures that, during tightening, a predetermined bending moment will be applied to the outer support rails of the lower plate.
During rotation to retract the pin, the wrap spring acts as a clutch by magnifying the rotational force applied to the pin. Because of the spring connection between the pin and the actuating knob, along with the slope of the spiral cam, only minimal rotational force is required to rotatably retract the pin.
Preferably, four of these pins are extended into engagement with the pallet to securely hold it in place at the workstation, with the four pins directed inwardly along a pair of spaced, parallel axes. At each point of engagement, an inwardly directed end of the pin and an engaged surface of the pallet are complementarily angled to provide precise and repeatable positioning of the pallet for machining operations.
According to one aspect of the invention, the pallets may be machined from a relatively lightweight aluminum and may be connected and disconnected in assembly line fashion at the workstation, with each of the pallets holding a workpiece. Because of the spiral cam and the slippable connection between the pins and their respective actuating knobs, this invention facilitates the automatic placement, connection and disconnection of the pallets at a workstation with a degree of precision within a ten thousandth of an inch, thereby facilitating accurate assembly line machining of multiple workpieces mounted on multiple pallets. The aluminum pallets may be automatically placed on the workstation, slid laterally into position by a robot and then accurately secured to the workstation automatically by using stepping motors to rotate the four pins into engagement with the pallet. If desired, the sequence of operation of the robot and the stepping motion may be controlled by a programmable computer.
Depending upon the shape and the configuration of the pallets, this invention also provides versatility in use. The bottom of the pallet may include a pair of spaced rolls, or rounded support members, for coacting with the pins so that the pallet may be used as a sine plate and hingedly raised at either end, as depicted and described in U.S. Pat. No. 4,927,125. Additionally, if the spaced mounting members are squared off on their bottoms and outer side surfaces, the pallet may actually be mounted vertically on either of its ends, perpendicular to the lower plate. In this position, a workpiece mounted to the pallet can be supported in a desired vertical plane. In effect, this enables the workpiece to be machined in three-dimensional space.
According to one preferred embodiment of the invention, four pallet connecting mechanisms are mounted to a lower plate which defines a workstation. The connectors are arranged so as to engage the pallet on two sides, along two spaced parallel axes. For each connector, the lower plate includes a mounting bore sized to receive a pin. The outer surface of the pin has a recessed spiral cam which cooperates with a radially directed cam follower rigidly connected to the lower plate and extending into the mounting bore. Upon rotation of the pin, the cam follower traverses the spiral cam and causes the pin to move axially with respect to the lower plate and to extend into or retract from engagement with the pallet.
This spiral cam extends once around the pin and includes an initial steeply inclined portion and a gradually inclined portion. Initially, rotation of the pin causes substantial axial movement while the cam follower traverses the steeply inclined portion of the spiral. However, after the pin has been rotated about 90.degree., the cam follower traverses the gradually inclined portion of the spiral, and the pin moves only slightly. When retracting the pin, the opposite occurs. The pin initially retracts slowly and then, in the last 90.degree., retracts substantially.
The forward or inner end of each pin has an angled surface which is machined to an angle which is complementary with an angled surface of an outer edge of a bore machined in the pallet. To connect the pallet, the pallet bores are aligned with the bores in the lower plate along the spaced axes, and the pins are extended inwardly into engagement with the pallet. The complementarily angled surfaces coact to assure that the pallet will be securely held in a precise position, within a minimal tolerance.
Each of the pins has a rearward, or outer end, which connects to an enlarged-diameter actuating knob. The knob facilitates rotation of the pin. More specifically, a wrap spring interconnects the pin and the actuating knob. The wrap spring has a radially extending tang which is held by the actuating knob and a plurality of coils which wrap around the outer surface of the pin. Depending upon which way it is desired to rotate the knob to affect extension of the respective pin, and whether or not the spring is wound with a left or right hand wrap, the tang may be either at the forward or the rearward end of the connector. The wrap spring has an inner diameter which is slightly less than the outer diameter of the pin, with the relative diameters depending upon the amount of torque wanted. Preferably, the inner surface of the wrap spring is flat to more securely engage the outer surface of the pin.
The wrap spring connects the actuating knob to the pin in a slippable manner. Because of the slippable interconnection between the actuating knob and the pin, the wrap spring acts as a torque limiting device when extending the pin. Thus, this invention eliminates the susceptibility of the pin to breakage when the pin is automatically rotated to extend and retract the pin into and out of engagement with the pallet. It also controls, to a preset amount, the force applied to the lower plate during extension which may cause rail deflection. The actuating knob may have an outer surface which is configured to facilitate manual grasping and turning to rotate the pin. The actuating knob may also include radially oriented holes sized to receive a radially inserted tommy bar to facilitate rotation of the pin about the axis when used manually. In an automated embodiment of the invention the actuating knob may be a toothed or beveled gear which can be easily connected to and automatically driven by a stepping motor.
One primary advantage of this invention relates to the use of a gear and a stepping motor to rotate the pin about 360.degree. to provide automatic, positive and precise engagement of the pin with the pallet. With four such pins engaging the pallet on opposite sides along a pair of spaced, parallel axes, as described above, extension and retraction of the pins may be fully automated with four stepping motors and a controller.
In addition to automating the extension and retraction of the connectors, the invention contemplates the automatic placement, positioning and then removal of a pallet from a workstation using four connectors. According to this embodiment, each of a first pair of spaced connectors on a first side of the workstation has a forward portion and a rearward portion, and the two portions are interconnected by an internal compression spring aligned along the engagement axis. For each of the first connectors, a tap screw extends internally along the outer portion, through the internal compression spring and is threaded into the forward portion.
The spring force causes the forward portion of the pin to extend forwardly along the axis away from the rearward portion. The actuating knob connects to the rearward portion, and the forward portion includes the angled surface adapted to engage the side of the pallet.
With all of the connectors retracted within their respective bores, a computer controlled robot arm places the pallet at the workstation at a position relatively close to a predetermined machining position. The actuating knobs of the connectors on the first side, i.e. the spring loaded side, are then rotated in the direction which would normally extend the pins. However, because the pallet blocks extension of these pins, the internal springs compress axially as the forward portions of the pins bear against the side of the pallet. Thus, this rotation "loads" the internal springs.
The pallet is then moved slightly laterally to align the tapered engagement surfaces of the pallet with the four connectors. This lateral movement allows the forward portions of the first pair of connectors to extend by spring force toward the pallet along the spaced axes and into engagement with the pallet on the first side.
Subsequently, the actuating knobs connected to a second pair of spaced connectors on a second, opposite side of the workstation are automatically rotated by stepping motors to extend the second pair of connectors into secure engagement with the opposite side of the pallet. The force applied by the connectors on the second side also holds the pallet against the rail or shoulder at the first side of the work station, with the same force applied each time because the force is controlled by the wrap springs. This method and structure connects the workpiece holding pallet securely to the workstation in a precise and repeatable position.
After a machining operation has been performed on the workpiece, the actuating knobs are again rotated 360.degree. to retract all four connectors from engagement with the sides of the pallet. The robot arm then picks up and removes the pallet, and another pallet is positioned at the workstation according to the same sequence of steps. By operatively connecting the robot arm and the stepping motors to a programmable controller, this entire sequence may be automated. If desired, the workstation may include an electronic eye, or other type of position detector to verify that the pallet is centered at the predetermined position prior to rotation of the actuating knobs to extend the connectors into engagement with the pallet.
According to another variation of the invention, the connectors may be used to engage a workpiece itself or a workpiece-holding pallet in three dimensional space, above a work surface or work table, with the connectors being mounted on pillars which are selectively positional on the work table. With this variation of the invention for supporting a workpiece or workpiece-holding pallet, a base plate or work table includes appropriately sized and positioned holes or recesses which receive the pillars.
With a plurality of such recesses, the pillars may be selectively positioned on the work table at locations suitable for holding a particularly sized workpiece or workpiece-holding pallet. These recesses may be machined in the work table to a desired shape, preferably with at least one V-shape, with the pillars having first, or bottom, ends complementary in shape for interfitting therein. This alternative is advantageous for some machining operations which require a degree of precision such that the pillars must be secured rigidly in place. With a work table having such machined recesses, angled set screw holes must be machined therein adjacent each recess, the set screw hole located opposite the V-shape. Set screws are then threaded through the holes and in to hole engagement with the pillars.
Alternatively, the recesses may actually be the standard positioning or mounting holes of a machine, such as the inverted T-slots of a standard Bridgeport milling machine. If that is the case, it is preferable then to mount the pillars at the intersections of the longitudinal and the transverse inverted T-slots.
Each pillar has a connector mounted therein, preferably oriented perpendicular to the axis of the pillar. For many workpieces or workpiece-holding pallets, four pillars are necessary, with four connectors associated therewith. However, the invention also contemplates using three of the pillar-mounted connectors for holding some workpieces or pallets. At least one of the connectors should be extendable and retractable along its longitudinal axis to facilitate engaged holding and subsequent disengagement of the workpiece or pallet with extension and retraction via application of a high, or controlled pressure to actuating knobs associated therewith. This may be done by using the wrap spring clutch described above. The other connectors should be spring-loaded for some extension and extraction.
The connectors may be used to suspend a workpiece of virtually any size or shape at a desired height above a work table, and in a desired orientation. This is done by locating the workpiece at the desired position, adjacent two or more spring loaded connectors which extend from pillars which have been suitably positioned or secured to the work table. At least one more, high-pressure, axially movable connector is also located adjacent the workpiece, due to suitable securement of its associated pillar to the work table. By axially extending the connector into contact with the workpiece, the workpiece can be engaged and rigidly held in three-dimensional space. By using the wrap spring clutch described above, this final holding step may be performed automatically, as by a robot, to facilitate automation of the entire workpiece holding process. After machining, the connector may be retracted, automatically if desired, to facilitate moving the workpiece from the held position.
Because of the ease in positioning the pillars to a work table at the selected positions, and the ability to axially move at least one connector into and out of secure engagement with a workpiece located in three-dimensional space, this invention simplifies set up operations for workpieces or pallets of various size and/or shape, while also providing a high degree of accuracy and repeatable accuracy. Additionally, because connectors are used, i.e. centers, this invention eliminates the need for clamps and other holding devices that are presently used but have been found to generate some of the above-described problems. With the need for such clamping devices eliminated, this invention also eliminates the problem of part distortion sometimes caused by performing a machining step on a part compressively held thereby.
For some workpieces, it is beneficial to keep the workpiece secured in place on a workpiece-holding pallet during a number of sequentially performed machining steps, and the pallet is moved from workstation to workstation. In this manner, multiple pallets are used to manufacture multiple parts, in assembly line fashion. This invention also accommodates this practice because the pillars and connectors may be used to hold a workpiece-holding pallet to which the workpiece is mounted. The pillars are simply arranged on the work table in a manner which accommodates the dimensions of the pallet. Since the multiple pallets each hold an identically shaped workpiece in an identical manner, the same high degree of accuracy and repeatability is achieved.
If a particular work table is to be used repeatedly for the same operation, but with workpieces or workpiece-holding pallets of different height, the same pillars may be used without the need for disconnection from the work table. This is accomplished by mounting the connectors on cartridges of varying height, wherein the cartridges are removably connectable to the pillars. With a plurality of connector-equipped cartridges available for interchangeable use on the pillars, workpieces or workpiece-holding pallets of different heights can be accommodated without having to move the pillars. This is particularly advantageous if the connector-equipped cartridges are relatively inexpensive compared to the pillars.
These and other features of the invention will be more readily understood in view of the following detailed description and the drawings.